ARM: at91: remove clock data in at91sam9n12.c and at91sam9x5.c files

As the CONFIG_OLD_CLK_AT91 option is gone, let's completely remove the AT91
old clock driver related data.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>